Like all early Meridian actives I've heard (M1s, M10s, M100s etc) my M10s have begun to hiss when idle (i.e. no signal). It's not noticable when music's playing so I've lived with it but now I'm wanting to sell them on so it's time to sort it out!

The hissing is a kind of pink noise and has subtle varations of pitch, seemingly random variations. It's almost like living near the coast with either a light wind blowing or soft waves on a shingle beach... not too unpleasant.

I've been told in the past that it's probably some capacitor current leakage but I don't know which capacitors they mean!

Is it likely the power supply capacitors or caps in the amp circuits / xover circuits?

They are active speakers with 2 x 2 channel A/B amps per speaker (2 channels for bass and one each for mid/treble) with each amp board having their own power supply (1 x torroid, diode bridge, 2 roederstein smoothing caps). For those that know them, I think they are the Meridian 103 circuits.

Hissing appears to be only in the mid/treble but this could be because it's filtered out by the low pass xover section.

Does anyone have experience with a similar thing with 26+ year old amplifiers?

ROE smoothing caps, hmm... there are some lines that have proven unreliable. Especially those with fancy plastic (?) casing, orange/red jobs, which are known to develop cracks if memory serves. In any case the suspect caps should be of the electrolytic kind. Could be coupling caps with excessive leakage (must be regular polar ones then, not the bipolar variety), could also be buffer caps with ESR gone through the roof (any hum?).

I'd try swapping the small electrolytics on one board and seeing whether that fixes it.